1. Field of the Invention
The present disclosure relates to a separator for a direct methanol fuel cell, and more particularly, to a separator for the separation of a gas/liquid mixture for a direct methanol fuel cell.
2. Description of the Related Art
A fuel cell is a galvanic cell converting the chemical reaction energy of continuously fed fuel and oxidant into electric energy. In general, a fuel cell comprises two electrodes separated by a membrane and/or by an electrolyte. The anode is surrounded by a flow of fuel, for example hydrogen, methane, or methanol, and the fuel is oxidized there. The cathode is surrounded by a flow of oxidant, for example oxygen, hydrogen peroxide, or potassium thiocyanate, which is reduced at the cathode. Depending on the type of fuel cell, the materials used to realize the single components are typically selected differently.
A direct methanol fuel cell (DMFC) is a low-temperature fuel cell which is operative in a temperature range as low as about 60-120° C. This type of fuel cell typically utilizes a polymer membrane as electrolyte. Methanol (CH3OH), with no previous reforming, is supplied directly to the anode together with water to be oxidized there. Carbon dioxide (CO2) is formed as waste gas at the anode. Atmospheric oxygen supplied to the cathode as oxidant reacts with H+ ions and electrons to form water. The advantage of the DMFC lies in the use of a liquid, easy-to-store, and very inexpensive source of energy, which can be distributed in plastic cartridges, for example. Moreover, a vastly branched infrastructure for methanol is already existing in many fields, e.g., through the use as an anti-freeze additive in windshield washer fluids for motor vehicles. Depending on the design, this type of fuel cell can provide power ranging from a few mW up to a few hundreds kW. Specifically, DMFCs are suitable for portable use as substitutes and supplements for conventional accumulators in electronic devices. Typical fields of use are in telecommunication and as power supplies of notebook computers.
The oxidation of methanol on the catalyst of the anode proceeds step by step, and several reaction pathways with various intermediate products have been proposed. To maintain high efficiency of the fuel cell, rapid removal of the reaction products from the region around the electrode is desirable. As a result of the temperatures encountered and the underlying chemistry, a liquid/gas mixture of CO2, water, water vapor, and non-reacted methanol is formed. Water and methanol should be recovered from this liquid/gas mixture so as to maintain self-sufficiency of the system for as long as possible, for example, by recycling as fuel. Furthermore, CO2 is desirably removed from the equilibrium, which is done by means of a CO2 separator. The CO2 is removed from this liquid/gas mixture, which is re-fed in the liquid fuel mixture to the anode, after adjusting the methanol concentration of the liquid fuel mixture. Separation of the gases is made by means of a CO2 separator.
Similarly, a liquid/gas mixture is forming at the cathode, comprising non-consumed air, water, and water vapor. To achieve long-lasting self-sufficiency of the system, as much water as possible is typically separated from the air and re-fed into the anode cycle. To this end, a heat exchanger is arranged downstream of the cathode outlet of the fuel cell so as to cool the mixture and achieve condensation of the water vapor.
Arranged downstream of the heat exchanger is an air separator separating the air stream from liquid water so as to re-feed the water into the anode cycle.
Accordingly, the separators are primarily used in water management and to remove CO2 from the equilibrium. Usually, the separators are put into practice in the form of separate units connected with the actual fuel cell via a feed line as is typical for such a liquid/gas mixture. The spatial distance also results in a temperature gradient, and water condenses from the gradually cooling liquid/gas mixture. Typical separators separate the phase mixture of liquid and gaseous or vaporous components, releasing the gaseous and/or vaporous components into the environment.
In a well-known manner, some typical separators used to separate the liquid/gas mixture includes a porous membrane. The inside of the porous membrane faces the liquid/gas mixture, and the outside thereof is in contact with the environment. Furthermore, such membranes are normally coated with and/or formed of hydrophobic materials. Diffusion channels extend from the inside to outside of the membrane, which are dimensioned such that (liquid) water situated on the inside cannot permeate, but gas can diffuse to the outside.
Some separators act to convey the liquid/gas mixture into a cavity adjacent to the gas-permeable membrane. The volume of the cavity and the relative position of the membrane depend on the orientation of the separator during operation and the volumes of liquid/gas mixture to be expected. The volume of the cavity is set in such a way that the liquid/gas mixture at an entry into the cavity can separate into a gaseous phase and a liquid phase over the entire volume of the cavity. The membrane is arranged so as to be adjacent to the top of the cavity, and contacts the gaseous phase during controlled operation. The liquid phase is discharged at the bottom. Sufficient functionality of such separators is maintained only in the correct spatial orientation of the separator. Tilting of the separator from its upright position should not be more than a few degrees at maximum so that the gaseous phase continues to contact the membrane. Precisely for mobile use of fuel cells, however, this circumstance represents a limitation.
US 2002/0192525 A1 discloses a CO2 separator for a DMFC, which operates independently of the separator's spatial orientation. The anodic waste gas of the DMFC enters a channel via an inlet of the separator, and the outlet opening is designed in such a way that pressure is built up as a result of entering waste gas. The channel has porous and hydrophobic walls permeable to CO2. The CO2 separator described can operate independently of its spatial position.